In optical information communication, optical fibers which serve as transmission media have been required to have larger transmission capacity as communications traffic increases. Particularly, a long-distance transmission optical fiber employing wavelength division multiplexing (WDM) has been keenly required to meet such requirement. Further, it is predicted that the communications traffic will continue to increase. Therefore, increase in the transmission capacity of the optical fibers is an urgent object to be attained.
In view of the circumstances, in recent years, in order to deal with the increase in the transmission capacity, there has been focused MDM (Mode Division Multiplexing) transmission that multiplexes a plurality of signals with use of an FMF (Few-Mode Fiber) capable of propagating a plurality of modes by superposing signals corresponding to each mode.
However, a mode dispersion inevitably occurs in the FMF. The mode dispersion reduces a transmission capacity in proportion to a transmission distance. It is therefore important to prevent the mode dispersion so as to manufacture a multimode fiber which can carry out a long-distance transmission with a large transmission capacity.
For example, it is known that a processing computational quantity of MIMO (Multiple-Input-Multiple-Output) employable in the MDM transmission is increased as a mode dispersion of the FMF is larger. Therefore, in a case where the mode dispersion of the FMF is large, processing of the MIMO may be delayed. As such, in the MDM transmission, an FMF having a small mode dispersion is needed. Further, because the MDM transmission is supposed to be used together with a WDM transmission, it is necessary to reduce a mode dispersion in a wide wavelength band.
In view of the circumstances, there is proposed a technique for compensating a mode dispersion in a light transmission path by combining a plurality of optical fibers having different mode dispersion characteristics. For example, the following Patent Literature 1 discloses a technique for expanding a transmission band width by compensating a mode dispersion not only in an optimized wavelength (e.g., 0.85 μm) but also in another wavelength (e.g., 1.3 μm) by combining two MMFs (Multimode Fibers).
The following Non-Patent Literatures 1, 2 disclose techniques for reducing a mode dispersion in a wider wavelength band by connecting two or more two-mode fibers in which symbols of mode dispersions and symbols of gradients of mode dispersions with respect to wavelengths are different from each other/one another.
In particular, the following Non-Patent Literature 1 proposes a light transmission path in which (i) a two-mode fiber obtained by applying a trench structure to a graded index core and (ii) a two-mode fiber only having a graded index core are connected to each other, and discloses that a mode dispersion having several ps/km or less can be achieved with use of the light transmission path in a C+L-band which is a wavelength band for optical transmission.
The following Non-Patent Literature 2 proposes a two-mode optical fiber having a stepwise refractive index distribution, and discloses that a light transmission path in which four optical fibers having respective adjusted parameters are connected to one another is used to achieve a mode dispersion having several ps/km or less in a C+L-band.